DSL technologies provide subscribers with high-speed access to computer networks over existing twisted-pair copper telephone wires. Competing DSL technologies include, for example, High-bit-rate DSL (HDSL), Rate Adaptive DSL (RADSL), and Asymmetric DSL (ADSL). Each of these DSL technologies offers different speeds, ranges, and operating characteristics.
One of the more promising DSL technologies is ADSL. A typical ADSL circuit connects an ADSL modem on each end of a twisted-pair of copper telephone lines. The ADSL modems communicate over three information channels: a high speed downstream channel, a medium speed upstream channel, and a Plain Old Telephone Service (POTS) channel.
In conventional ADSL systems, the POTS channel is split off from the other channels by a splitter installed at the customer's premise. The installation is typically done by technical personnel, and may include the re-wiring of existing telephone lines. This kind of installation can incur significant costs and inconvenience for the customer.
For new DSL systems intended to be deployed in large volumes to the general public, it is desirable to provide for easy installation of the ADSL modem at the customer's premise. One way to simplify installation is to remove the splitter entirely from the system. The ADSL modem can then be plugged directly into an existing telephone jack. This splitterless configuration enables the ADSL modem to share the same wiring as the customer's telephone sets without the need for installing a splitter or rewiring the customer's premise.
The deployment of ADSL systems in a splitterless environment prompted the development of splitterless ADSL modems. One example of a splitterless ADSL modem is defined in International Telecommunication Union Standardization Sector ("ITU-T") recommendation G.992.2 (October 1998), which is incorporated by reference herein in its entirety. With splitterless ADSL modems, the installation of the splitter and/or rewiring is no longer necessary. Without the splitter, high frequency ADSL signals are transmitted directly to the customer's telephone sets. The ADSL signals include downstream signals transmitted from the Central Office (CO) modem and upstream signals transmitted from the Customer Premises Equipment (CPE) modem.
High frequency ADSL signals have a higher transmit power than POTS signals and may be electronically "clipped" by internal telephone set circuitry if such power exceeds a predetermined power threshold designed to protect the user's hearing. This is especially problematic for upstream signals since the CPE modem is typically located close to the telephone set. Thus, the upstream signal level at the telephone set is basically equal to the transmitted signal level, while the downstream signals are attenuated by the telephone lines. This "clipping" effect can introduce nonlinear distortion within the POTS frequency band, which can interfere with POTS services (e.g., telephony, voiceband modems, and facsimile machines).
A fast retrain procedure for estimating the proper level of transmit power for ADSL modems in a splitterless environment is described in ITU-T recommendation G.992.2. During this fast retrain procedure, a single R-line-probe signal is transmitted to the telephone sets from the CPE modem to estimate the proper transmit power level. The characteristics (i.e., shape and level) of this R-line-probe signal is vendor-discretional, and thus not defined in G.992.2.
Unfortunately, estimating the proper transmit power is difficult because splitterless environments can include several different types of telephone sets, each connected to the same telephone wiring as the CPE modem. These telephones sets typically have different electrical characteristics, thereby making it difficult for the CPE modem to set the proper transmit power level without prior knowledge of the types of telephone sets being used. Depending on the types of telephone sets used, if the transmit power level is reduced too much, the reduction of the signal-to-noise ratio (SNR) of the ADSL signals may cause significant data rate reduction, or even become inadequate for data communication. On the other hand, if the transmit power is reduced too little, distortion may still be audible to the user.
Accordingly, there is a need for a system and method of controlling the transmit power level of a DSL modem operating in a splitterless environment without regard to the number and type of telephones sets being used. The system and method should minimize audible distortion in the voiceband while providing a signal-to-noise-ratio (SNR) adequate for DSL data communication. Such a system and method should operate on the fly without prior knowledge of the electrical characteristics of the telephone sets being used.